Wetness detectors are commonly used in clinical analyzers to detect the extent to which patient sample has been dispensed onto a slide-like test element, hereinafter "slide", for analysis. Examples are disclosed in U.S. Pat. No. 4,420,566. Threshold values can be set to cause the detector to flag any slide on which insufficient liquid is dispensed.
Although such a detector has been quite useful, it has not been capable of reliably detecting whether or not a "pre-spot" condition has occurred. That is, the detector has been designed heretofore to detect the desired drop, and not the possible error condition of a "pre-spot". A "pre-spot" occurs if and when the dispenser accidentally drops a quantity of liquid onto the slide prior to the desired dispensing time. If the pre-spot is large enough, e.g., is more than approximately 10% of the desired volume at the time of dispensing, this will adversely affect the slide performance when actual dispensing occurs.
Attempts have been made to use the wetness detector of the aforesaid patent to detect such prespotting. This has been based on an examination of the R-C time decay curve produced by the sensor circuitry. That is, the exponential decay should approach asymptotically an intermediate value, for example 250 A to D "counts". However, any pre-spotting will interrupt the gradual decay of this curve, and the computer can be programmed to detect such interrupts.
Although the concept is clear, in practice it has been difficult to set an absolute value of change in the A/D signal that should represent a pre-spot. The reason is that any absolute value tends to be a function of a particular analyzer. Even the first derivative (i.e., a decrease of say, 24 A/D counts), has not been satisfactory, because of the "noise" that can artificially produce such an effect. More precisely, noise can exist in such analyzer, mostly due to physical motion of the test element relative to the detector, that occurs in the system, which does not represent a pre-spot condition at all.
Thus, prior to this invention it has not been possible to use the wetness-detector to detect pre-spotting, with any high degree of accuracy that will catch all prespots without flagging non-prespots as errors.